The present invention relates to the manufacture of hollow porous fibers produced from a liquid polymer solution, and particularly to the quenching of those fibers in a quench bath following their extrusion.
Quench baths are known, as in U.S. Pat. No. 3,199,148 to Koppehele. Extruding of polymer filaments is known, for example, from U.S. Pat. No. 3,0894,384 to Denves et al. Fiber extrusions are also known from U.S. Pat. No. 4,154,856 to Akin.
The production of hollow porous fibers, i.e., microporous, and in particular skinless nylon microporous filter fibers, is also well known. See, for example, U.S. Pat. No. 3,876,738 to Marinaccio and Knight and U.S. Pat. No. 4,604,208 to Chu et al., the entire disclosures of which are incorporated herein by reference.
The production of hollow porous fibers involves extruding a hollow tube of liquid polymer solution, referred to as dope, into a chemical quenching solution, which causes the hollow porous fiber structure to form. During the extrusion, the dope emerging from the extrusion die, referred to as a spinneret, is essentially still in liquid form. It gradually solidifies over time while exposed to the chemical quenching solution held in a quench bath.
If a nascent, not yet solid, hollow fiber is bent or otherwise distorted while it is still predominantly in its liquid form, the cross section of the fiber will become permanently distorted and it will no longer have the concentric circular inner diameter and outer diameter cross section which it had as it was emerging from the spinneret.
It is desired to maintain a concentric, circular cross section for the hollow fiber. It is known that one way for achieving that objective is to design the chemical quench bath to maximize the vertical distance directly beneath the spinneret through which the fiber can descend. This provides the fiber with the longest possible time to form and solidify, before the fiber is finally made to change direction by being led around a reel, roll or other guide and thereafter exit the quench bath horizontally. Obviously, a very deep or tall quench bath is desired, especially in making large diameter fibers, that is those having an outer diameter of 30 mils or more. Very deep baths have practical problems, however. These include large bath volume and relative difficulty in gaining access to the bottom area of the bath for maintenance or to retrieve items that may have fallen into the bath. The need for large bath volume arises because once the fiber is redirected, e.g. around guides, it moves over a generally horizontal path to the bath exit and the deep bath must also be wide enough to provide the horizontal path.